Occasional memory lapses like forgetting where the car is parked are not signs of early Alzheimer's, but doctors still aren't entirely sure how much forgetfulness indicates a greater risk of developing the full-blown disease.

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The problem isn't that you can't remember where you put your keys from time to time, it's that once you find the keys, you still can't recall that you put them in that spot. The problem isn't forgetting a single appointment, but a pattern of blanking on important events or responsibilities.

Or, as Dr. Barry Gordon of the Johns Hopkins Memory Clinic puts it: "Miss an exit on the highway once, that happens. Miss it five times and that’s another story."

As part of the explosion of research into Alzheimer's disease over the last decade, scientists have become more aware of the differences between typical age-related memory lapses and a more serious condition called mild cognitive impairment.

People with mild cognitive impairment may be more forgetful than usual, but can still pay the bills and handle most daily tasks. Signs include losing track of a conversation, difficulty remembering details from a TV show they've just watched or consistently forgetting appointments.

"These people are starting to forget important information that they used to remember regularly, like doctor appointments or meeting friends," says Dr. Ron Petersen, a member of the Alzheimer's Association's medical and scientific advisory council.

Alzheimer's is thought to be the underlying cause of most mild cognitive impairment, but not everyone who experiences it develops dementia or worsening symptoms, research indicates.

Dementia is estimated to affect about 10 percent of people over 65. Of those patients, about 65 percent have Alzheimer's and 15 percent have cardiovascular problems like hardening of the arteries or stroke that can impair the mind. The rest have various uncommon conditions.

Because it can be difficult to recognize the boundaries between typical absentmindedness, mild cognitive impairment and the early stages of Alzheimer's, people who are worried about recurring forgetfulness should consult a physician. There are tests to check a person's mental abilities, and neurologists can determine if something is seriously wrong.

Being aware of your memory lapses is probably a sign that the problems isn't serious, says Gordon. The time to worry about Alzheimer's is not just when you think your memory is getting spotty, but when your friends or family start to notice your forgetfulness.

"Those that worry about it most are the least likely to have it," he says. "In general the disease robs people of their ability to appreciate that they have a memory problem."